This invention relates to polyesters and specifically polyesters having a controlled degree of crosslinking obtained by the reaction of dicarboxylic acid anhydrides with epoxides.
It is well known that linear polyesters are obtained as a result of the reaction of an epoxide with a dicarboxylic acid anhydride. Early work on this subject is described in an article by R. F. Fischer in Journal of Polymer Science Vol. XLIV, pp. 155-172 (1960). In that article, Fischer examined the reaction of epoxides such as allyl glycidyl ether, epichlorohydrin, propylene oxide and vinylcyclohexene mono-epoxide with anhydrides such as phthalic anhydride and maleic anhydride to produce linear polyesters. He found that the reaction was catalyzed by tertiary amines and quaternary salts, that the molecular weight of the polyester could be controlled by the presence of active hydrogen-containing compounds such as alcohol and water and that the reaction is independent of monomer concentration.
A similar reaction is described in U.S. Pat. No. 2,966,479 which describes heating a polybasic acid anhydride with a monoepoxide in the presence of a catalyst which is a tertiary amine or a quaternary ammonium salt, under substantially anhydrous conditions and at a temperature of above 70.degree. C. A continuous multistage process for producing linear polyesters by this type of reaction, but using a different catalyst system and staged addition of the epoxide, is disclosed in U.S. Pat. No. 3,723,390.
In addition to the large amount of art on such linear polyesters, there is much art on the reaction of polyepoxides with dicarboxylic acid anhydrides to produce high crosslinked materials. This art is exemplified by U.S. Pat. No. 3,692,715 which describes a heat curable epoxy resin composition obtained by reacting a polyglycidyl ether of a polyhydric phenol r a polyglycidyl ester of an aromatic or cycloaliphatic polycarboxylic acid with a dicarboxylic acid anhydride using as a catalyst an aliphatic carboxylic acid salt of a polyvalent metal.
A useful reference book on highly cross-linked epoxide polymers (commonly known as epoxy resins) is "Epoxy Resins" by I. Skeist and G. R. Somerville (Reinhold, 1958).
Commercial epoxy resins are obtained by a variety of processes which include homopolymerization of a polyepoxy compound and reaction of such a compound with a mirror proportion, usually about 6-15% by wt., of a curing agent that is conventionally a polyamine, such as diethylene triamine, or dimethylaminopropylamine, or 30-40% by wt. of a polyamide resin, or 35-40% by wt. of an anhydride such as phthalic anhydride. It will be appreciated that where an anhydride is used, the product is substantially a polyester.
The difference between the reactions producing linear and crosslinked polyesters can be more fully appreciated by reference to the following reaction formulae by which each may be prepared. The anhydride common to both is maleic anhydride though this is only for the sake of illustration, and it is understood that the maleic anhydride could be wholly or partially replaced by a different cyclic acid anhydride. ##STR1## where R is a monovalent hydrocarbyl radical.
It will be obvious that, in theory at least, a monoepoxide reacted with an anhydride will produce a linear product having an --A BA B AB-- type structure. However, in practice it is necessary to take steps to avoid or minimize consecutive addition of epoxy units which gives rise to polyether segments. In addition when maleic anhydride is used special techniques must be used to avoid crosslinking via the double bond of the anhydride. Polyether segments usually lead to soft, rubbery polymers and cross-linking via the double bonds of the maleic anhydride gives rise to polyesters that are black weak foams (the result of partially decomposition of anhydride) with no commercial value.
Cross-linked If on the other hand, a di-terminal epoxide is used, the reaction will proceed as follows: ##STR2## where R' is a divalent hydrocarbyl radical. The reaction will, according to accepted theory therefore, produce one cross-linkage for every di-epoxide molecule reacted.
As is predictable, the properties of these linear and highly cross-linked products are very different. The properties of linear polyesters vary according to the epoxide used. With alkylene oxides such as ethylene oxide the polyesters are usually soft and tacky particularly when an excess of epoxide is used but with e.g. styrene oxide, the polyesters tend to be harder and more brittle but have little dimensional strength. The anhydride used has a similar effect in that polyesters made using (e.g.) succinic and glutaric acid anhydrides tend to have lower polymer melt temperatures than those made using maleic or phthalic anhydrides. The crosslinked polyesters on the other hand are very hard and have good dimensional strength.
Linear polyesters obtained by the above process and incorporating an unsaturated anhydride such as maleic anhydride have found significant commercial applications in the form of solutions in styrene which are copolymerized in the presence of glass fibers to provide the well-known fiberglass-type products.
Cross-linked polyesters of the type described above have found extensive use as adhesives (the so-called "epoxy" resins) and to some extent as molding materials, though these tend to be of limited use in view of the great ridigity and a tendency to brittleness that they display. An even greater disadvantage is the high cost of the polyepoxy component which makes such polyesters extremely expensive.
It has now been found that apart from producing highly crosslinked polymers with great thermal stability, it is possible to produce a relatively low-cost polyester with a controlled amount of cross-linking that retains its thermoplastic character and has a very useful combination of properties making it suitable for a variety of applications for which both the linear and cross-linked polyesters are totally unsuited.
Surprisingly, it has been found that there is no simple relationship between the physical properties of the polymer and the amount of cross-linking, and an area intermediate the extremes has been defined in which the polymers have excellent properties without some of the deficiencies apparent with either wholly linear or fully cross-linked polymers. The type of product obtained can, moreover, be tailored to end-uses ranging from adhesives through foams and polymer concretes to molding compositions.